Wide availability, relative inexpense, and functional versatility have led to the adaptation and use of polyurethane elastomers with specific physical properties in a diverse array of technical applications. One such application is as conductive and semi-conductive members in image forming devices, such as electrophotographic printers. As printer art evolved according to the motivation of increased office space efficiency in light of ergonomic considerations, the need has arisen to develop more compact image forming devices having a smaller device footprint. As a consequence, there is a need for reduced-geometry elastomeric printer components. However, the necessary size adaptations alter the functioning of components with respect to, for example, required compressive forces, nip widths, low compression set, low hardness, triboelectric charging capacity, and rotational torque. Utilization of reduced-geometry components having the same properties as the originally-sized components results in the occurrence of unacceptable print variations and distortions in the final printed product.
In electrophotographic printer applications, a developer roll typically functions to provide a layer of toner to a charged photoconducting drum. The toner is metered onto the surface of the developer roll via a doctor blade. The electrical properties of the developer roll assist in the electrostatic transfer of the charged toner from the roll surface to the photoconductor drum. Examplary compositions and properties of conventional developer rolls are disclosed in detail in U.S. Pat. No. 5,707,743 to Janes et al., U.S. Pat. No. 5,804,114 to Janes et al., U.S. Pat. No. 5,874,172 to Beach et al. and U.S. Pat. No. 6,117,557 to Massie II, et al., all of which are herein incorporated by reference. These prior art patents relate to rolls with high surface resistance, processes of manufacturing high surface resistance rolls involving the oxidation of polybutadiene in the presence of ferric chloride, antioxidant agent components, and the desirability of caprolactone ester polyurethane developer rolls, respectively. These patents also disclose the print performance advantages and electrical properties of two-layered developer rolls and techniques for production of semiconductive rolls in general, and the particular techniques for the production of the resistive surface layer characteristic of the present inventive rolls.
Typically, plasticizers are used to reduce the hardness of polymeric elastomers such as cast urethanes. However, plasticizing additives, which are not chemically bonded into the polymer matrix, and are therefore free to migrate throughout the roll, including to the surface, may cause problems in electrophotographic print applications. At the surface of the developer roll, these migrating additives interact with the toner particles, causing the toner particles to undesirably coalesce on the surface of the developer roll.
It is known in the polyurethane art that addition of polyols to a polyurethane mixture may lower the hardness of the final polyurethane elastomer. However, these polyols typically result in not only a decrease in hardness, but also concomitant undesirable changes in other physical properties of the elastomer.
A common technique to produce a semi-conductive roll with a resistive layer is to prepare a core using any standard rubber molding technique, such as casting liquid urethanes or rubber transfer molding. The core is then ground to the correct dimensions and either spray or dip coated with a resistive material to the desired thickness. The coating is usually applied in several layers to build up the desired thickness. The production of such rolls suffers from higher process costs and defects which are typical problems with such methods of manufacturing a roll having a high resistive layer.
Hence, it is desirable to provide an elastomer roll exhibiting acceptable hardness via means that do not involve additives with the potential to migrate, and which do not cause unacceptable changes in other physical properties. In addition, it would be advantageous to provide a roll which eliminates non-uniformities in print performance caused by geometry reduction of the roll.